Performance of the Prototype Detector

The project and its progress using a full scale prototype TPC has been documented at several conferences [22,23] and in a progress report [3] submitted to PSI (BVR31) in June 2000. In the table below we show the timeline of milestones achieved so far.

date	milestone
Jan 1997	proposal R-97-05 presented at BVR
Aug 1997	progress report on systematics and chamber tests
	conditional approval granted
Dec 1998	first beam test with prototype TPC + 4 MWPC's,
	new DAQ based on VME boards, analog readout
March 1999	continuation of test run with new digital readout (2 TDC400)
Dec 1999	test run with second prototype TPC and 6 MWPC's,
	full digital readout with 10 TDC400
March 2000	continuation of test with upgraded DAQ,
	event selection, 3 thresholds tested
Dec-March 1999	development of gas purification and chromatography at 0.01 ppm level
April 2000	setup of high purity gas detector system and protium production
December 2000	full protium production operational, design of final setup

For the test runs at the $\mu$E4 beam, 2 full-sized TPCs ( $\Delta x\cdot\Delta y\cdot\Delta z$ = 15 x 8 x 30 cm$^3$) and 4-6 MWPC's were manufactured in Gatchina. These assemblies were mounted in a cylindrical pressure vessel (l = 50 cm, d = 32 cm) filled with 10 bar purified hydrogen gas and exposed to a low energy (35 MeV/c) muon beam at fluxes 1-50 kHz. Fig. 10 presents a sideview of the improved setup used during the experimental test runs in December 1999.

Figure 10: Setup of the second full prototype TPC with 6 wire chambers for muon and electron tracking (sideview, z-y plane).

A muon telescope at the beam entrance and two electron scintillation telescopes acted as $\mu$/e trigger and timing devices. The signals from scintillators, TPC anodes, TPC strip cathodes and MWPC's were fed into 10 custom designed ``TDC400'' VME64 units developed at PSI for dead-time free high-rate data acquisition (chapter 3.2). Two x-y muon chambers (PC1, PC2) allowed entrance tracking of the muons up to the TPC, while the two x-chambers (PC4, PC5) and z-chambers (PC3, PC6) above the TPC were used for reconstruction of the electron tracks.

The detector performance was satifactory and proved that stable chamber operation in pure hydrogen is feasible. Nevertheless, a rather high chamber voltage was used, about 7 kV on 2-4 mm spacing, in order to obtain a sufficient gas amplification for electrons ($\sim$5000). Since some cross talk of the heavy ionizing slow muons is unavoidable, this may cause some unwanted correlations. This was one of the incentives, that we have chosen for the final setup to detect and track the electrons by a totally independent system outside the TPC vessel.

A 0.3 mm thick Mylar entrance window of 40 mm diameter was mounted in the beam axis, thin enough that most of the 35 MeV/c muons at intensities 20-50 kHz stopped inside the hydrogen volume (Fig. 12). A muon scintillator telescope in front of the pressure vessel provided good muon timing and pile-up information.

Two double scintillator telescopes located above and below the pressure tank (e1,e2: 30x20 cm$^2$, d=2 mm, solid angle $\sim 10\%$) detected the electrons from muon decay. Telescope e1 was placed directly in line with PC's 3-6 and was therefore an ideal trigger for electron tracking with these chambers. Since using scintillators produced flawless and undistorted measurements of the $\mu e$ time, we adopted this solution also for the final setup (scintillator array).

The main data was recorded by the TDC400 units at a clock rate of 5 MHz. The hits of all detectors were stored for contiguous time regions of $\sim$10 ms, providing the full history information around individual muon stops. A short time slice from this time region is displayed in Fig. 11.

Figure 11: Event display showing a 60 $\mu$s time slice from the contiguous readout buffer. The central panel (y-z plane) shows TPC anode signals as function of time (s). Muons are distinguished from electrons by using two discriminator levels. The upper panel (y-x plane) indicates the corresponding information from the strip cathodes. The bottom panel shows the times of various detector hits. The right panels indicate wires hits on muon (upper) and electron (lower) MWPC's, respectively. Three stopping muons and their decay electrons and one through going particle are seen in the display, the third muon producing after $\sim 5 \mu s$ a decay electron seen by the electron wire chambers.